Fabrics or fiber webs made of or containing cellulose esters and/or ethers, and in particular cellulose acetate and cellulose triacetate, (all the foregoing hereinafter collectively called CA) can undergo a thermal bonding treatment to thereby cause the CA fibers of the fabric or web to bond to one another and/or to other fibers in the fabric or web. In addition, when two or more layers of a CA containing fabric or web need to be "joined" or "bonded", such layers can be bonded together by a thermal process known in the art as "calendering". Generally, such calendering processes are carried out on material having a low moisture content. The calendering process is performed by passing the fabric or web between a pair of rollers which exert a pressure on the fabric. Typically, one or both of the rollers is heated to a selected temperature to accomplish the calendering. A calendering process for applying a coating to a fabric is described in the Encyclopedia of Polymer Science and Engineering, Vol. 6 (Wiley-Interscience, John Wiley & Sons, New York 1986), pages 639-640 and ibid., Vol. 2, pages 606-622. A calendering process to bond fibers of a material to one another and/or to fibers made of other materials are carried out in a similar manner using calender rollers heated to a selected temperature appropriate for the fiber(s) to be bonded. In the calendering process, the combination of the temperature and the pressure exerted by the rollers causes the fibers to soften and/or melt, and to bond to themselves or to other fibers in the fabric or material being calendered.
Many natural and artificial fibers show little tendency to bond with the application of heat, or require such high temperatures for bonding that a fabric made of such fibers is detrimentally effected if calendering is attempted. For example, fabrics made of cellulose acetate must typically be calendered at a temperature of about 220.degree. C., or higher, in order to effect fiber bonding. For example, U.S. Pat. No. 2,277,049 disclosed a calendering temperature of 232.degree. C. (450.degree. F. at page 3, left column, line 68). However, at these temperatures, the fibers may be degraded with impairment of fabric quality (e.g., discoloration, strength degradation, fiber embrittlement and similar impairments). For example, cellulose acetate fabrics may be used in apparel where their draping qualities are desired. Embrittlement of the fibers stiffens the fabric and the draping quality is lost. In addition, high temperature bonding results in greater energy costs, bonding non-uniformity from roller distortion, and imposes high machinery maintenance costs, for example, seals, bearings, roller distortion caused bowing of the rollers at their center due to the heats required which results in the need for more frequent roller replacement, and similar items.
A number of patents describe methods which have been used in various processes to improve the bonding of cellulosic fibers. U.S. Pat. No. 2,692,420 to Bamber et al. (Bamber) describes the production of felts from a combination of (1) wool or other animal fibers capable of felting and (2) cellulose acetate fibers. Banber recites the difficulty encountered in preparing such felts and teaches treating such felt mixtures with organic softening agents to soften the cellulose acetate fibers before felting, and thereby facilitating the felting process.
U.S. Pat. No. 2,277,049 to Reed (cited above), in addition to disclosing a calendering temperature of 232.degree. C., also discloses the use of various organic solvents to soften binding fibers in a fabric which is a mixture of binding fibers and cotton. Reed finds the use of such solvents is objectionable. Additionally, Reed also discloses water wetting a fabric that has been heat calendered in order to soften the still calendered fabric. This water wetting is post-calendering and does not influence the calendering temperature employed.
U.S. Pat. No. 5,783,39 to Duckett et al describes the use of acetone vapor to lower the softening temperature of cellulose acetate fibers in order to lower calendering temperature. While lower bonding temperatures may be achieved using the method of Duckett et al., the use of acetone vapors creates a fire and explosive hazard which is not desirable in commercial operations.
U.S. Pat. No. 2,673,163 to Rohm describes adding water to bulk cellulose esters such as cellulose acetate flake in order to lower the melting point of the ester prior to extruding it through an orifice such as in a melt spinning process. The amount of water incorporated in the molten mass is above the normal "regain" moisture of the dry ester, typically in the range of 0.5-10%. "The preferred moisture content is at 8-9% with a hydrolyzed cellulose acetate derivative, and at about 6% with cellulose triacetate." (See Rohm, column 2, lines 2-5.)
Cellulose acetate fibers are known to bond at temperatures of about 220.degree. C. The use of organic softening agents such as described by Bamber can lower this temperature, but the use of such organic substances with acetate fabric is undesirable from worker safety, both economic and environmental considerations. Other than through the use of organic chemical plasticizers, the art generally does not teach a satisfactory method of reducing the temperature at which cellulose acetate or cellulose triacetate fibers can be bonded to one another in a fabric or to fibers of other materials which may be present in a fabric or web. In the present invention, water is acting as a plasticizer.
Accordingly, it is an object of this invention to provide a process whereby the temperature required to bond cellulose ester and cellulose ether fibers to one another or to fibers of other materials present in a fabric or web can be reduced.
It is a further object of this invention to provide a process using water to reduce the temperature required to bond cellulose ether and cellulose ester fibers to one another or to fibers of other materials present in a fabric or web.
It is an additional object of this invention to provide a process whereby calendering a fabric or a web containing fibers of a cellulose ester or a cellulose ether at selected temperatures and in the presence of a selected amount water will cause the cellulose ester/ether fibers to bond to one another or to fibers made of other materials at temperatures less than 220.degree. C.; and will further impart a range of physical characteristics to the calendered fabric, for example, "hand " or "stiffness".